No, just a domain. The term "subdomain" is just kind of confusing. Any Fully Qualified Domain Name [FQDN] is registered with a Registrar and thereafter known as an FQDN domain. Domains can be hosted and delegated [which is common and perhaps close to the idea of a subdomain, but still, it's a domain].

So, if a user has more than one domain, you want some kind of login or auth that asks for simply domainname which that user can type over in a dialogue box, either a form on some login website or a popup. I think every ISP or other that hosts such sites has a login form that asks for the domainname. You could simply go to such a site and view the source, or save the source, and see how they do it.

If you mean the Windows login prompt, that is a Windows study on how to allow login choices with multiple domains. Microsoft must have some documentation on that somewhere, but i've no idea where it is. We do, in fact, have multiple domains, yet Windows Server is not geared toward easily telling one how to add entries to the domain list drop down.

Consider the following scenario: You are working on a website and make something great - something that lets the server work with information submitted by your users.
This could be anything, from a simple guestbook to a e-Money solution. But what…

Foreword (July, 2015)
Since I first wrote this article, years ago, a great many more people have begun using the internet. They are coming online from every part of the globe, learning, reading, shopping and spending money at an ever-increasing ra…